The present invention relates to a charging system for charging a set of constituent batteries, and more particularly to a charging system for a battery assembly consisting of a plurality of single battery cells connected in series.
One conventional example of the battery charging circuit is shown in Japanese Patent Provisional (Unexamined) Publication 61-206179. To equalize the charging conditions of the individual cells, a zener diode is connected in parallel to each cell in a reverse bias current direction. This charging system is arranged to bypass the charging current and thereby to adjust the progress of charging by utilizing a break down of the zener diode in accordance with an increase of the terminal voltage of each cell.
In this conventional example, however, the zener voltage must be chosen preliminarily to suit a charge termination voltage of each cell, and, in order to prevent overcharge, the charging current must be bypassed completely without flowing through the cell. Therefore, the bypass circuit of the zener diode must have a large capacity especially for boost charge (quick charge) or for a large capacity battery. As a result, the conventional charging system requires additional measures for dissipation of heat due to the bypass current, and incurs cost increase. Furthermore, the arrangement of a zener diode of a large capacity for each cell increases the size of the charging system.